When bending in the open, using press brakes, especially when bending metal sheets, the accuracy of the bending angle which is obtained depends, firstly, on the characteristics of the sheet, such as its thickness, which may vary from one sheet to another, and its physical and geometrical characteristics, on which the elastic return of the sheets depends, and secondly on characteristics of the press itself, especially tolerances in the construction of the machine and the tooling and wear on the tools, and also deformations appearing in the press at the instant of bending.
The majority of the press brakes which are at present in use necessitate a manual adjustment of the press before the bending and/or during the bending.
An optical safety device for press brakes is known from U.S. Pat. No. 4,166,369. The said device comprises a plurality of light projectors mounted on one side of the press and forming a plurality of horizontal light beams received on a plurality of light receivers mounted on the other side of the press, and being adapted to stop the operation of the press brake when any of said light beams are interrupted by an operator's hand or finger. The only function of said device is safety. Determination of bending angle of the workpiece to be bent is not possible with said device.
Optical means for detecting a bending angle of the work are known from both Japanese patent applications Nos. 59 160707 and 59 160708, using the light reflected of a light beam projected perpendicularly against the one part of the workpiece. Such means only permit the local measure of the bending angle, but variations of the bending angle along the workpiece cannot be detected. Furthermore, said means are ineffective in case of asymmetric bending.